Vernier controls, as referred to herein, are those controls which provide both coarse and fine adjustments. Several types of vernier controls are known in the art. One type includes a variable resistor having a circular resistance track and a conductive brush that is coupled to inner and outer concentric operating shafts. These shafts rotate to move the brush along the resistance track to vary the resistance between the brush and the ends of the track. The outer shaft is coupled to the brush through a gear reduction module to provide a fine adjustment, and the inner shaft is coupled directly to the brush to provide a coarse adjustment. In this type of control the coarse and fine adjustments are coupled together through the gear reduction module so that the operation of one shaft will result in the turning of the other shaft as well. Such a control is disclosed in Slagg et al., U.S. Pat. No. 3,676,822, issued July 11, 1972.
In Peart, U.S. Pat. No. 4,075,597, issued Feb. 21, 1978, a control is disclosed for selectively coupling a single operating shaft to a single resistance track, which is disposed on a substrate, through a pair of alternately operable drive gears spaced along the longitudinal axis of the operating shaft. A contact-carrying rotor is mounted on the substrate, and has gear teeth for engaging whichever drive gear is operated. This patent also discloses a two-resistor module having a pair of these controls disposed side by side in a housing with their respective substrates spaced apart and substantially parallel to one another.
Another type of control that provides both coarse and fine adjustments includes two variable resistors spaced axially along a single operating shaft that is coupled to both resistors. At least two different controls belong to this category. One of these controls relies on a phenomenon known as "backlash." Backlash is a degree of looseness or play in a mechanical coupling mechanism that allows a drive shaft to be turned a rotational distance before it drives a mechanical output member. A control that employs backlash has front and rear potentiometers. The rear potentiometer has an arcuate resistance track with a higher resistance value than an arcuate resistance track included in the front potentiometer, usually by ratio of about 10:1. The low resistance potentiometer has a brush which rides along its resistance track and is directly coupled to an operating shaft. The high resistance potentiometer has a brush which rides along its resistance track, but is coupled to the operating shaft through a backlash coupling mechanism. The brush in the rear potentiometer will be allowed to "idle" while the operating shaft is rotated to move the brush in the front potentiometer along its resistance track for a fine adjustment. The mechanical independence of the brush in the front potentiometer can be 40.degree., for example, before the brush in the rear potentiometer is picked up by the shaft and moved for a coarse adjustment.
Another vernier control in which a single shaft is coupled to two variable resistors also has front and rear resistive elements formed in annular tracks and spaced axially along a concentric operating shaft. The rear element has a high resistance value for coarse adjustment and the front resistance element has a low resistance value for fine adjustment. The operating shaft is normally positioned for operation of the front potentiometer for fine adjustment. The operating shaft is also coupled, however, to the rear potentiometer through a vernier clutch drive. The operating shaft can be depressed, which decouples it from the front potentiometer, and allows it to be coupled to the rear potentiometer for coarse adjustment.
Multiple adjustment controls are used in, among other things, color television sets, for brightness, color and contrast adjustments. In this application it is desirable to have a coarse adjustment which is set by a trained technician at a factory or service center, and which is not easily accessible to the user, and a fine adjustment which can be operated by the user. The controls described above do not provide this capability because the coarse and fine adjustments are equally accessible, being controlled by a pair of concentric shafts or by a single shaft.
In the field of electrical controls there is a continual demand for components which are smaller in size, and less expensive. A vernier control that does not need a vernier drive, and that employs a single module instead of a pair of potentiometer modules will produce economic advantages in manufacturing that are deemed apparent.